1. Field of the Invention
The present invention relates to a printer printing an image with a line head and particularly relates to a printer characterized by having a line head controller for controlling the line head.
2. Description of the Related Art
A thermosensitive printer (thermal printer) is a widely used printer type. The thermal printer applies current according to gradations (image tones from white to black) to the head elements of a head, thereby transferring a thermal-melting or thermal-sublimating material coated on a ribbon to a medium by the heat of the head elements or coloring a thermosensitive material coated on a medium by the heat of the head elements.
FIG. 1 shows one example of the constitution of a line head controller which is a circuit provided to control current applied to the respective head elements of the line head in a conventional thermal printer for printing an image by the line head. In this line head controller (to be referred to simply as head controller hereinafter) 51, N pieces of image data for one line among the image data represent gradations at the positions of dots on the respective lines of a medium width, for example, eight bits (i.e., with 256 density levels), respectively. If the number of dots per line is, for example, 2056, then 2056 image data are written to a line memory 52.
A gradation counter 53 is a counter which counts up gradation data from a minimum value 0 to a maximum value 255 of 256 gradations each represented by eight bits, with a clock signal CK used as an operating clock, after the counter 53 is reset at an initial value 0 by a reset signal RS.
First, when the value of the gradation counter 53 is 0, image data are read out one by one from the line memory 52 and a comparator 54 compares each of the image data with the value of the gradation data. When the value of the image data is higher than the value of the gradation data, a H (high) signal CP is outputted from the comparator 54 and when lower, an L (low) signal CP is outputted from the comparator 54.
The signals CP each indicating a comparison result as well as shift pulses (not shown) are sequentially fed from the head controller 51 to a sift register 62 in the line head (to be referred to simply as head hereinafter) 61. As a result, by the time the comparator 54 completes comparison for all the image data for one line read from the line memory 52, signals indicating whether the density levels of the image at the respective dot positions on this one line are higher than 0 are stored in the shift register 62 in the head 61.
When the comparator 54 completes comparison with respect to all the image data corresponding to one line and read from the line memory 52, the head controller 51 allows the signals stored in the shift register 62 in the head 61 to be outputted simultaneously and to be latched by a latch circuit 63 in the head 61. The respective signals latched by the latch circuit 63 are supplied to head elements in the head 61.
FIG. 2 shows one example of the structure of the head element. Transistors 65 are arranged on a substrate (not shown in FIG. 2) so as to correspond to the respective positions of dots on one line (therefore, the number of transistors 65 is N, equal to the number of dots per line), and each of the transistors 65 functions as one piece of the head element. The collector of each transistor 65 is connected in parallel to a semiconductor 66 through a resistor 64. Both ends of the semiconductor 66 are connected to a power terminal 68 using a copper wire 67. The emitter of each transistor 65 is connected to an earth terminal 70 using a copper wire 69.
Each of the signals latched by the latch circuit 63 shown in FIG. 1 is supplied to the base of each transistor 65 at the position of a corresponding dot. Thus, current is applied only to transistors 65 to the bases of which H signals are supplied (which transistors 65 are located at positions corresponding to the positions of dots having density levels higher than 0) and only these transistors 65 are heated.
Next, in the head controller 51, the gradation counter 53 counts up gradation data by one. Image data are read again from the line memory 52 one by one and each of the image data is compared with the value 1 of the gradation data by the comparator 54. When the comparison is completed, the signals stored in the shift register 62 in the head 61 are latched anew by the latch circuit 63 in the head 61. By doing so, current is applied only to transistors to which bases H signals are supplied (which transistors 65 in this case are located at positions corresponding to the positions of dots having density levels higher than 1) and only these transistors 65 are therefore heated.
Thereafter, the head controller 51 continues the same processings repeatedly, while the gradation counter 53 counts up gradation data one at a time up to a maximum value 255. As a result, H signals (i.e., PWM signals which density levels are modulated to pulse widths) are supplied to the bases of the respective transistors 65 for a period of time according to gradations at their corresponding dot positions. Therefore, current flows for a period of time according to the gradations. A thermal-melting or thermal-sublimating material coated on a ribbon is transferred to a medium by the heat of the respective transistors 65 (or a thermosensitive material coated on the medium is colored as a result of heat generated at the transistors 65 due to the application of current to the transistors 65), whereby an image corresponding to one line is written on the medium.
After the image corresponding to one line is written, the gradation counter 53 is reset at the initial value 0 in the head controller 51 and image data corresponding to the next line are written to the line memory 52. The above-stated steps are conducted repeatedly for these image data.
In the meantime, according to the conventional head controller 51 as stated above, while the value of the gradation data at the gradation counter 53 is close to a minimum value 0, H signals CP are outputted from the comparator 54 for almost all image data read from the line memory 52. During this period, therefore, current is simultaneously applied to almost all transistors 65 of the head 61.
As stated above, if current is simultaneously applied to almost all transistors 65, high current flows through the semiconductor 66 and the copper wire 67 (a resistor common to the transistors 65) correspondingly. If the number of transistors 65 is 2056, current flowing through the common resistor (to be referred to as xe2x80x9ccommon resistancexe2x80x9d hereinafter) is, for example, 8 to 10 amperes. If such high current is applied to the common resistance, the following disadvantages occur.
(1) Generally, in a thermal printer, the length of time for allowing the latch circuit 63 of the head 61 to latch signals (i.e., the length of time for applying current to the transistors 65) is corrected within a range of a cycle in which the gradation counter 53 counts up gradation data, in accordance with the number of image data for which H signals CP are outputted from the comparator 54 of the head controller 51, thereby suppressing the occurrence of uneven printing derived from a voltage drop at the common resistance (particularly the voltage drop at the semiconductor 66), which correction will be referred to as xe2x80x9cnumber correctionxe2x80x9d. If current as high as 8 to 10 amperes is applied to the common resistance, however, the voltage drop becomes larger at the common resistance. Thus, even if this number correction is made, it becomes impossible to suppress the occurrence of uneven printing.
(2) The period of time for which current flows through almost all transistors 65 is a period for which the value of the gradation data at the gradation counter 53 is close to a minimum value 0. For that reason, although the period is only part of the time needed for printing images for one line, it is required to provide a large-sized, expensive power supply capable of supplying current as high as 8 to 10 amperes to the head 61 for that period.
(3) Due to a large voltage drop at the common resistance, efficiency for converting the electric energy of the power supply into the heat energy of the transistors 65 deteriorates. Here, if the resistances of the individual head elements are increased, current applied to each head element decreases, thereby making it possible to decrease the current flowing through the common resistance even in a period in which current is simultaneously applied to almost all the head elements. However, developing a head having high-resistance head elements disadvantageously makes the design and manufacturing processes complex and costly.
The present invention has been made in view of the above-stated disadvantages and the object of the present invention is to overcome the disadvantages including the occurrence of uneven printing, the need to provide a large-sized power supply, high cost and the deterioration of energy conversion efficiency resulting from the application of high current to the common resistance of a line head. Thus, instead of developing a line head having high-resistance head elements, an available line head is used in a thermal printer for printing an image.
To obtain the above object, the applicant of the present invention proposes a line head controller of a thermal printer printing an image by a line head, for generating gradation data representing image gradations by stages; alternately selecting each image data the number of which corresponds to the number of head elements of the line head, and data of value 0, and switching selection as to which data is to be selected, the image data or the data of value 0, every time a value of the gradation data changes; and feeding a signal indicating a comparison result of comparing the selected data with the gradation data to the line head.
In this line head controller, for the number of image data corresponding to the number of the head elements of the line head, each image data and the data of value 0 are alternately selected and the selected data is compared with the gradation data. Then, for each image data, selection is switched as to which data is to be selected, the image data or the data of value 0, every time the value of the gradation data changes.
Accordingly, in any periods in which the gradation data has a given value, the data of value 0 is selected and compared with the gradation data for half the image data among the image data the number of which corresponds to the number of the head elements. Thus, the signal indicating that the value of the image data is higher than the value of the gradation data, is not fed to the line head.
Consequently, in the thermal printer provided with this line head controller, the number of the head elements applied with current becomes about half the number of the head elements in the printer provided with the conventional line head controller such as the head controller 51 shown in FIG. 1 (i.e., the total current flowing through the respective head elements is reduced to about half the current in the conventional case) throughout the periods in which the gradation data have all values (i.e., the entire printing time for one line).
Therefore, even in a period in which the value of the gradation data is close to the minimum value, current is not simultaneously applied to almost all the head elements but applied only to approximately half the head elements, so that the maximum value of the total current flowing through the respective head elements is reduced to a half of the current in the conventional case.
As a result, current flowing through the common resistance of the line head becomes greatly lower than the current in the conventional case, while using the same head as the conventional line head without newly developing a line head having high-resistance head elements. This can overcome disadvantages including the occurrence of uneven printing, the need to provide a large-sized power supply, high cost and the deterioration of energy conversion efficiency as described in (1) to (3) above.
The individual image data are compared with every other value of the gradation data and two image data corresponding to adjacent two head elements are compared with gradation data different from each other. Accordingly, an image composed by images printed by adjacent two head elements (i.e., by images located at the positions of two adjacent dots) is printed with the same gradations as that in case of comparing the image data with all the values of the gradation data. It is, therefore, possible to prevent the gradations from appearing coarser when viewing a printed image.
It is noted that the line head controller is more preferably constructed to switch the selection per line as to which data is to be selected, the image data or the data of value 0, for the respective image data with the gradation data being the same value.
By doing so, the selection is switched every line as to which data is compared with the gradation data, each image data or the data of value 0 with regard to the respective image data corresponding to the same head element. Due to this, not only an image composed by images printed by adjacent two head elements (i.e., images located at positions of two adjacent dots in a line direction) but also an image printed on adjacent two lines by the same head element (i.e., images located at positions of two adjacent dots in cross direction) can be printed with the same gradations as those in case of comparing the image data with all values of the gradation data. It is, therefore, possible to further prevent the gradations from appearing coarser when viewing a printed image.
Further, it is more preferable that the line head controller is constituted to feed a signal correcting current-carrying time for carrying current to head elements of the line head to the line head in accordance with the signal indicating the comparison result. By doing so, it is possible to more suppress the generation of uneven printing resulting from voltage drop at the common resistance of the line head.
Next, the applicant of the present invention proposes a printer printing an image by a line head, comprising a line head controller for controlling generating gradation data representing image gradations by stages; for alternately selecting each image data the number of which corresponds to the number of head elements of the line head, and data of value 0, and switching selection as to which data is to be selected, the image data or the data of value 0, every time a value of the gradation data changes; and for feeding a signal indicating a comparison result of comparing the selected data with the gradation data to the line head.
This printer is provided with the above-stated line head controller according to the present invention. Therefore, if the printer is a thermal printer, the disadvantages including the occurrence of uneven printing, the need to provide a large- sized power supply, high cost and the deterioration of energy conversion efficiency as stated in (1) to (3) above are overcome, and the gradations are prevented from appearing coarser if viewing a printed image.
In case of the printer as in the case of the above, it is more preferable that the line head controller is constituted to switch the selection as to which data is to be selected, the image data or the data of value 0 with the gradation data being a same value per line for the respective image data, and to feed a signal correcting current-carrying time for carrying current to the head element of the line head to the line head in accordance with the signal indicating the comparison result.
Further, the applicant of the present invention proposes a line head controller of a thermal printer printing an image by a line head, comprising: gradation generation means for generating gradation data representing image gradations by stages; selection means for alternately selecting each image data the number of which corresponds to the number of head elements of the line head, and data of value 0, and for switching selection as to which data is to be selected, the image data or the data of value 0, every time a value of the gradation data changes; and comparison means for feeding a signal indicating a comparison result of comparing the selected data with the gradation data to the line head.
According to this line head controller, the disadvantages of the thermal printer including the occurrence of uneven printing, the need to provide a large-sized power supply, high cost and the deterioration of energy conversion efficiency as stated in (1) to (3) above are overcome, and the gradations are prevented from appearing coarser if viewing a printed image.
In case of the printer as in the case of the above, it is more preferable that the line head controller is constituted to switch the selection as to which data is to be selected, the image data or the data of value 0 with the gradation data being a same value per line for the respective image data, and to further comprise voltage drop correction means for feeding a signal correcting current-carrying time for carrying current to the head elements of the line head to the line head in accordance with the signal indicating the comparison result.
Next, the applicant of the present invention proposes a thermal printer printing an image by a line head, comprising a line head controller, the line head controller comprising: gradation generation means for generating gradation data representing image gradations by stages; selection means for alternately selecting each image data the number of which corresponds to the number of head elements of the line head, and data of value 0, and for switching selection as to which data is to be selected, the image data or the data of value 0, every time a value of the gradation data changes; and comparison means for feeding a signal indicating a comparison result of comparing the selected data with the gradation data to the line head.
According to this line head controller, the disadvantages of the thermal printer including the occurrence of uneven printing, the need to provide a large-sized power supply, high cost and the deterioration of energy conversion efficiency as stated in (1) to (3) above are overcome, and the gradations are prevented from appearing coarser if viewing a printed image.
In case of the printer as in the case of the above, it is more preferable that the line head controller is constituted to switch the selection as to which data is to be selected, the image data or the data of value 0 with the gradation data being a same value per line for the respective image data, and to further comprise voltage drop correction means for feeding a signal correcting current-carrying time for carrying current to the head elements of the line head to the line head in accordance with the signal indicating the comparison result.
Next, the applicant of the present invention proposes a printer printing method of printing an image by a line head, comprising: a gradation generation step of generating gradation data representing image gradations by stages; a selection step of alternately selecting each image data the number of which corresponds to the number of head elements of the line head, and data of value 0, and of switching selection as to which data is to be selected, the image data or the data of value 0, every time a value of the gradation data changes; and a comparison step of feeding a signal indicating a comparison result of comparing the selected data with the gradation data to the line head.
According to this printing method, if the printer is a thermal printer, the disadvantages including the occurrence of uneven printing, the need to provide a large-sized power supply, high cost and the deterioration of energy conversion efficiency as stated in (1) to (3) above are overcome, and the gradations are prevented from appearing coarser if viewing a printed image.
In case of this printing method as in the case of the above, it is more preferable that in the selection step, the selection as to which data is to be selected, the image data or the data of value 0 with the gradation data being a same value is switched per line for the respective image data, and that the printing method further comprises a voltage drop correction step of feeding a signal correcting current-carrying time for carrying current to head elements of the line head to the line head in accordance with the signal indicating the comparison result.